Friction welding has been employed for many years to weld heat-fusible weld members together. The process involves bringing the members to be welded together under pressure, and thereafter spinning one of them rapidly enough so that sufficient frictional heat is generated to cause fusing or melting of the weld members at their interface. Rotation is thereupon stopped, and the melted material at the interface is allowed to solidify, thereby providing a high-quality welded joint.
The process has distinct advantages over other welding processes. For instance, friction welding can be used to join materials that are adversely susceptible to oxidation, since the contacting surfaces effectively seal the fused area from contact with surrounding air. Furthermore, heat is present only in the immediate area of the joined surfaces, thus tending to eliminate possible adverse heat effects in adjacent areas of the members. The friction processes can also be employed to weld materials submerged in liquids, or to seal vessels containing liquids. Particularly in the case of plastics, because of their poor heat conductivity, frictionally created heat increases the temperature in the weld zone swiftly, making rapid welding and high production rates possible. An additional advantage of the process includes the fact that no hazardous solvents or adhesives are required to join the members together. Furthermore, friction welding equipment is reasonably inexpensive, and capable of volume production. It is also relatively uncomplicated, lending itself to piecework welding, even with ordinary shop machinery.
While friction welding processes offer many advantages, they suffer from the fact that measurement of the variables affecting the quality of the welds produced is difficult. In the absence of such data, fundamental welding information necessary to obtain an understanding of the process variables, and their interrelationship, is difficult if not impossible to obtain. Furthermore, without such information, process variables cannot be controlled during welding so as to achieve optimal welds, nor can welding techniques be easily transferred from one type of equipment to another. Also without such information, it is difficult to extrapolate conclusions based on experience with one material, to the welding of different materials.
In regard to the measurement of process variables, the coefficient of friction of the materials being welded has an important bearing on the amount of heat generated during rotation of the weld members; therefore, its determination under actual dynamic welding conditions is of particular interest. In order to measure dynamic friction, however, it is necessary to accurately measure the torque applied at the interface of the members being welded, simultaneously with the application of the normal forces urging the members together. Unfortunately, in ordinary friction welding apparatus the necessary connection between the force-generating component of the apparatus and the weld members being joined entails substantial frictional contact that interferes with accurate torque measurements, and therefore, with the accurate determination of dynamic coefficients of friction.
In addition, inertial forces of the drive assemblies of other friction welding apparatus tend to result in "over-run" rotation of the rotating weld members attached to the assemblies, causing the generation of shearing forces in the weld zone during solidification of the molten material. Such forces tend to create poor quality welds as a consequence of the highly stressed welds resulting therefrom.
Furthermore, the value of frictional coefficients depends upon the constantly changing temperature of the welding surfaces during their frictional engagement. This fact makes it difficult to determine coefficients by alternative test methods under substantially adiabatic conditions similar to those encountered during friction welding. Thus while other tests to determine frictional coefficients are known, they are commonly unable to provide proper temperature correlation.
ASTM test procedure D1894, for example, involves frictional measurements at the surface interface between a "test sled", and a moving plane. As the plane moves past the sled, however, the face of the sled is constantly being cooled, due to its contact with the ever-changing surface of the moving plane which is at the ambient temperature. This prevents valid friction measurements under dynamic temperature conditions.
An alternative test procedure entails a rotating disk in association with a rod or cylinder that contacts the disk at an interface with a measured force. Like the prior procedure described, however, the point of contact constantly encounters fresh surfaces at the ambient temperature of the apparatus, thus preventing accurate substantially adiabatic friction measurements.
In view of the preceding, therefore, it is a first object of this invention to provide an improved friction welding apparatus.
A second object of the invention is to provide a friction welding apparatus that allows process variables to be continuously monitored during the welding procedure.
An additional aspect of this invention is to furnish a friction welding apparatus that provides means for imposing normal forces on the members to be welded through a substantially frictionless coupling.
Another aspect of this invention is to provide accurate torque measurements throughout the friction welding procedure.
A further object of this invention is to provide a friction welding apparatus that permits the measurement of dynamic coefficients of friction.
Yet an additional aspect of this invention is to make available a friction welding apparatus that can be stopped substantially instantaneously at a desired moment in the welding process.
Still another aspect of this invention is to provide a frictionless welding apparatus that facilitates measurement of friction coefficients under substantially adiabatic heating conditions.
Yet a further aspect of this invention is to allow the gathering of sufficient friction welding data to carry out computer simulation, and to generate desirable process control parameters to guide the operation of other, different friction welding apparatus.